The present invention relates to a plant for liquefying natural gas. More specifically, a pre-cooled, dual heat exchanger, dual refrigerant system. Such a plant comprises a natural gas pre-cooling heat exchanger having an inlet for natural gas and an outlet for cooled natural gas and a liquefaction heat exchanger comprising a first hot side having an inlet connected to one outlet for cooled natural gas and an outlet at the top of the liquefaction heat exchanger for liquefied natural gas. The plant further comprises a pre-cooling refrigerant circuit for removing heat from the natural gas in the natural gas pre-cooling heat exchanger, and a liquefaction (or main) refrigerant circuit for removing heat from natural gas flowing through the first hot side of the main heat exchanger. Such a plant is for example known from International patent applications publication No. 96/33 379 and publication No. 97/33 131. The latter publication further discloses that the compressors in the pre-cooling refrigerant circuit and in the liquefaction refrigerant circuit are mechanically interconnected.
During normal operation, the natural gas to be liquefied is pre-cooled in the hot side of the natural gas pre-cooling heat exchanger by heat exchange with refrigerant evaporating in the cold side. Evaporated refrigerant is removed from the cold side of the heat exchanger. This evaporated refrigerant is liquefied in the pre-cooling refrigerant circuit. To this end the refrigerant is compressed in a compressor to an elevated pressure, and the heat of compression and the heat of vaporization are removed in a condenser. The liquid refrigerant is allowed to expand in the expansion device to a lower pressure, and at this pressure the refrigerant is allowed to evaporate in the cold side of the natural gas pre-cooling heat exchanger.
The pre-cooled natural gas is subsequently further cooled, liquefied and sub-cooled to about its atmospheric boiling point in the first hot side of the liquefaction heat exchanger by heat exchange with refrigerant evaporating in the cold side of the main heat exchanger. Evaporated refrigerant is removed from the cold side of the liquefaction heat exchanger. This evaporated refrigerant is liquefied in the main refrigerant circuit. To this end the refrigerant is compressed in a compressor to an elevated pressure and the heat of compression is removed in a number of heat exchangers. The refrigerant is then condensed and separated into a light, gaseous fraction and a heavy, liquid fraction, which fractions are further cooled in separate hot sides in the liquefaction heat exchanger to obtain liquefied and sub-cooled fractions at elevated pressure. The sub-cooled refrigerants are then allowed to expand in expansion devices to a lower pressure, and at this pressure the refrigerant is allowed to evaporate in the cold side of the main heat exchanger.
This plant is usually called a single-train liquefaction plant. Such a plant is so designed that the maximum amount of gas that can be liquefied is practically limited by the maximum amount of power that can be delivered by the turbines driving the compressors in the pre-cooling and the main refrigerant circuit. In order that more natural gas can be liquefied a second train of the same size is built. A plant consisting of two such trains is called a double-train liquefaction plant. The double-train liquefaction plant, however, will have a liquefaction capacity that is twice the liquefaction capacity of the single-train liquefaction plant. Because such a large increase of liquefaction capacity is not always required, there is a need to get an increase in the liquefaction capacity of about 40 to about 60%.
This about 40 to 60% increase of liquefaction capacity can be achieved by turning down the production of the double-train liquefaction plant to the desired level. Alternatively this aim can be achieved with two smaller trains, each having a maximum capacity of about 70 to 80% of the larger train.
It is an object of the present invention to provide a plant for liquefying natural gas having a liquefaction capacity which is 40 to 60% higher than that of the larger liquefaction train, wherein the building expenses are less than the building expenses associated with a plant consisting of two smaller trains, each having a maximum capacity of about 70 to 80% of the larger train.
To this end the plant for liquefying natural gas according to the present invention comprises one natural gas pre-cooling heat exchanger having an inlet for natural gas and an outlet for cooled natural gas, a distributor having an inlet connected to the outlet for cooled natural gas and having at least two outlets, and at least two main heat exchangers each comprising a first hot side having one inlet connected to one outlet of the distributor and an outlet for liquefied natural gas, which plant further comprises a pre-cooling refrigerant circuit for removing heat from the natural gas in the natural gas pre-cooling heat exchanger, and at least two main refrigerant circuits for removing heat from natural gas flowing through the first hot side of the corresponding main heat exchanger, wherein the pre-cooling refrigerant circuit further comprises at least two additional circuits for removing heat from the main refrigerants in each of the main refrigerant circuits.
The invention will now be described by way of example in more detail with reference to the accompanying drawings, wherein